1. Field of the Invention
This invention relates to a narrow-band interference filter, and more particularly to such a filter which comprises a plurality of alternating reflective layers arranged in a series in the form of layers on a substrate, and having a construction pattern (HL).sup.2 H with spacing layers LL arranged between the alternating reflective layers, where the H layers have a high index of refraction and the L layers have a low index of refraction and the same optical thickness as the H layers.
2. Description of the Prior Art
In optical communications transmission systems having guided waves, the output of a communications link should have the least possible reactive effect on its input in order to keep the interference in the entire system as small as possible. When a narrow-band interference filter is provided in an optical communications transmission system, it is also necessary that the light signals should not be reflected on the filter at the beginning of a line.
In accordance with the observations set forth by H. A. Macleod in his book "Thin Film Optical Filters", A. Hilger Ltd., London, 1969, practically reflection-free interference filters can be constructed from dielectric layers arranged on a substrate if dielectric matching layers are additionally interposed, which layers are generally arranged both between the filter layer system and the substrate, and also between the filter layer system and the medium from which radiation emanates.
In practice, this is the equivalent of coating the entire filter layer system. As a matched interference filter of this type consists only of dielectric, non-absorbent layers, it is symmetrical in respect of its optical properties, i.e., it possesses infinitesimal reflection on both sides.